When (like committed linnets) I     
   With shriller throat shall sing
The sweetness, Mercy, Majesty,
   And glories of my King;
When I shall voice aloud how good
   He is, how Great should be,
Enlargèd Winds, that curl the Flood,
   Know no such Liberty.

The first four lines of stanza 3 speak of occasions when Lovelace expresses his devotion to Charles I, his beloved King. Lovelace was imprisoned in 1642 at the outbreak of the religiously motivated Civil War against King Charles I. Lovelace had spoken out on Charles's behalf in Parliament thus winning the enmity of the Puritan "Roundhead" rebels. It was from prison that Lovelace wrote this lyric poem letter to his beloved.

In these opening lines, Lovelace compares his expressions of devotion to King Charles I (who was later beheaded by order of Cromwell), through the poetic device of a simile, to the songs of linnets. Linnets are finches of the Old World, as European birds are classified, and are small brown song birds. In other words, Lovelace says his praise of Charles is the song of the forest birds (I've traded Lovelace's simile for a metaphor!).

When (like committed linnets) I     
   With shriller throat shall sing
The sweetness, Mercy, Majesty,
   And glories of my King;

Lines 5 and 6 reiterate his mention of occasions when he shall speak "aloud" of Charles's goodness and greatness.

When I shall voice aloud how good
   He is, how Great should be,

In the final two lines of this iambic octave stanza, with alternating tetrameter and trimeter, Lovelace provides the analogy that expresses how much liberty is generated in his experience by these praises. He says that unfettered (i.e., unbound) enraged storm winds that sweep across the ocean and rampage over the waves know not the liberty that he experiences. In other words, when praising King Charles like song birds in a forest singing of how good and great the King is, Lovelace has more freedom and liberty than the liberated raging ocean wind.     

When I shall voice aloud how good
   He is, how Great should be,
Enlargèd Winds, that curl the Flood,
   Know no such Liberty.